Guiding Options

NIRI standalone (i.e., without AO) requires the use of a peripheral wavefront sensor for both imaging and spectroscopy. PWFS2 is preferred over PWFS1, as it can be used on fainter guide stars, can run at a higher frequency for a given guide star, works better under windy and cloudy conditions, and is smaller and thus vignettes less of the field of view.

For NIRI imaging at f/6 the PWFS2 guide star should be located >5' from the target For spectroscopy and for imaging at f/14 and f/32 a somewhat smaller inner radius than 5' may be used (down to about 4.25'). For stars closer than 5' the position editor in the Observing Tool should be inspected carefully to check that the PWFS probe does not block the science field. The outer limit for PWFS2 guide stars can be displayed on the OT position editor and is discussed here.

Due to difference in flexure between the PWFSs and NIRI, the NIRI field can (especially when the airmass is changing rapidly) shift on the array by as much as 0.3-0.5 arcsec in one hour. In order to maintain the science target in the slit during long integrations, one should break up the observing sequence into roughly 1-1.5 hour sections and reacquire the target in the slit after each section. One also should align the slit EW whenever scientifically viable, as it has been shown that during the tracking of the target most of the drift of the target is along the slit.

NIRI with ALTAIR is used in two modes: with a natural guide star (NGS) or with the laser guide star (LGS).

In NGS mode the guide star (which may be the target itself) is used to provide wavefront information for the deformable mirror as well as overall image motion for tip/tilt.

In LGS mode the laser provides information for the deformable mirror while a nearby star must be used for tip/tilt. This star can be much fainter than an NGS star used with the deformable mirror.

For more detailed information on the performance of the PWFS and ALTAIR, including guide star magnitude limits consult the PWFS and the ALTAIR web pages.